1. Field of the Present Invention
The present invention relates generally to electromagnetic field tomography and spectroscopy, and in particular, to the non-invasive functional imaging, detection and mapping of electrical excitation of a biological tissue with the help of electromagnetic field tomography and spectroscopy using a sensitive material (solution), injected into the biological tissue or in the circulation system, that is characterized by having dielectrical properties that are a function of the electrical field generated by biological excited tissue. The invention includes several versions of the systems differentiated on the basis of multiple frequency, polarization and type of sensitive material (solution) utilization. Further, the invention includes computer implemented software specifically configured and tailored for the system and method for non-invasive detection and mapping of electrical excitation of biological tissue with the graphical and three-dimensional tomographic imaging interface.
2. Background
It has long been known that within the electromagnetic spectrum, biological tissues have different electrical/dielectrical properties, and consequently, visual images of such tissues may be produced based on these properties. For example, it is known that the dielectrical properties of tissues with high (muscle) and low (fat and bone) water content are significantly different. During the last decade the changes in the dielectrical properties of tissues caused by various physiological and pathological alterations have been intensively studied. It has been demonstrated that dielectrical properties of malignant tumors and normal tissues are different in breast, lung, colon and liver. It has also been demonstrated that ischemia, infarction and hypoxia change the dielectrical properties of myocardial tissue. The amount of those differences (i.e., the contrast in their dielectrical properties) varies with frequency, type of tissue and the presence and type of disease, and the magnitude of variation may range from a few percentage points up to 4–5 times between the normal tissue and the diseased tissue. These examples demonstrate a high potential for the use of electromagnetic tomography in biomedical applications.
As a result, microwave tomography has been in the scope of interest of research groups for several years. For example, research by at least some of the present inventors has resulted in U.S. Pat. Nos. 5,715,819, 6,026,173 and 6,333,087 for microwave tomographic and spectroscopic systems and methods for detection of physiological and pathological conditions of biological tissues and physiological imaging of such tissues. The entirety of each of these patents is incorporated herein by reference.
Unfortunately, the research and development of this technology for biomedical applications has also met with significant difficulties. One such difficulty is the high attenuation of electromagnetic fields within the body. Attenuation is less at lower frequencies, but unfortunately, lower frequencies also result in lower spatial resolution. The compromise between attenuation and spatial resolution forms a frequency optimum for microwave imaging. J. C. Lin theoretically estimated that the frequency spectrum from 2 GHz to 8 GHz is the optimum for microwave imaging of biological tissue. Our estimations suggest that microwave imaging of whole scale biological objects with reasonable acquisition time and spatial resolution of 6–8 mm can be performed at frequencies near 1 GHz. Of course, experimentally achieved spatial resolution cannot compete with the spatial resolution achieved in X-ray imaging, simply because of the large difference in wavelength.
However, the possibility of imaging physiological and pathological conditions of tissues, highlighted earlier, makes this technology promising. For example, it has been determined that microwave tomography and spectroscopy are capable for detection of changes in myocardial blood supply, tissue hypoxia, myocardial ischemia and infarction, i.e. functional imaging. Research by at least some of the present inventors has experimentally proved that a tissue's dielectrical properties are a sensitive indicator of its functional and pathological conditions and the degree of such changes is large enough to be tomographically imaged.
FIG. 1 is a graphical representation of the changes of myocardial ε″ following short time 20%, 40%, 60% and 100% blood flow reduction. The changes of myocardial ε″ following short time 20%, 40%, 60% and 100% blood flow reduction. Summarized data for group of seven canines are presented as normalized on the baseline values. Data for three frequencies (0.2 GHz, 1.1 GHz, and 6.0 GHz) are expressed as mean +/− SD.
FIGS. 2A & 2B are graphical illustrations of the spectral changes in myocardial permittivity ε′ and resistance ρ, respectively, during 10% hypoxia. The percent difference from the mean baseline data, summarized for the group of 7 canines, is shown. The bar graph inserted into the bottom right of FIG. 2A represents group averaged changes in arterial blood pH and pO2.
FIGS. 3A and 3B are graphical illustrations of the spectral changes in myocardial permittivity ε′ (A) and resistance ρ (B) during 2 hours acute ischemia. The percent difference from the mean baseline data, summarized for a group of 6 canines is shown.
FIG. 4 is a graphical illustration of the changes in myocardial dielectric properties (ε@) for 2-week-old canine myocardial infarction. Summarized data for a group of five canines are presented as mean percent change +/− SD in dielectrical values from normal zones of the infarcted hearts. The values are compared with fresh tissue from a 10-year-old human post-infarction aneurysm.
FIGS. 5A & 5B are reconstructed electromagnetic tomographic images of excised canine heart (longitudinally view through the long axis base to apex for ε′-top and transversal view through an area with significant infarction injury for ε″-bottom) together with anatomical slices. The frequency is 1 GHz, and the scales are in centimeters.
However, microwave tomography and spectroscopy do not appear to be capable of detecting changes in the dielectrical properties of myocardium, caused by a spread of electrical excitation in tissue. Preliminary studies, conducted in tissue bath using cardiac excited tissue and electromechanical uncoupling pharmaceutical agents, have indicated that the dielectrical properties of cardiac excited tissue change during the excitation cycle. The exact degree of such changes is unknown at present time, but it appears to be less than 1 percent. This is a relatively small variation in tissue dielectrical properties to be reliably reconstructed using modern electromagnetic tomographic technologies. Further, it is difficult to acquire the necessary data during the short period of time available during the circulation cycle.
Thus, previous approaches to localizing the origin of such phenomenon as cardiac arrhythmias have depended on one of three principal techniques: catheter mapping, electrical excitation mapping during cardiac surgery, or body surface mapping of electrical potentials and magnetic fields. Each of these techniques has limitations. For example, catheter mapping and excitation mapping during surgery are inherently invasive, provide only limited access, and are time sensitive. On the other hand, body surface mapping can be performed in a non-invasive, low-risk manner, but with such poor definition that the data is generally considered unsuitable for directing subsequent therapy. Thus, a need exists for a non-invasive system by which electrical excitation of a biological tissue may be reliably detected and mapped.
Another difficulty faced in the use of microwave tomography for the purposes described hereinabove is the wave character of the distribution of the electromagnetic field within and around a body. These lead to a highly complicated image reconstruction theory, i.e. the problem of diffraction tomography. The linear, ray approach applicable to X-ray tomography does not work properly with regard to microwave tomography. Thus, a need exists for advanced, non-linear diffraction approaches to the process of image reconstruction.